Unable to create new virtual directory in SSRS report manager

Trying to change URLs

from:

    http://blah/ReportServer 

to:

    http://blah/ReportServer/Foo

Error: Microsoft.ReportingServices.WmiProvider.WMIProviderException: The virtual directory specified is not valid. Make sure the specified name is not too long and doesn't contain illegal characters. (example: ;)

---> System.Runtime.InteropServices.COMException (0x8004020D): Cannot modify or delete an object that was not added using the COM+ Admin SDK (Exception from HRESULT: 0x8004020D) --- End of inner exception stack trace --- at Microsoft.ReportingServices.WmiProvider.RSWmiAdmin.ThrowOnError(ManagementBaseObject mo) at Microsoft.ReportingServices.WmiProvider.RSWmiAdmin.SetVirtualDirectory(String application, String virtualDirectory) at ReportServicesConfigUI.WMIProvider.RSReportServerAdmin.SetVirtualDirectory(UrlApplication app, String virtualDirectory)

Changing to:

    http://blah/ReportServer_Foo 

works just fine. Not sure what's going on. Any information is appreciated.

Looks like its following the convention of SERVERNAME_INSTANCENAME used commonly with integrated authentication for example.